Genus Boswellia as a new candidate for neurodegenerative disorders.

Neurodegenerative diseases, characterized by progressive loss of neurons, share common mechanisms such as apoptotic cell death, mitochondrial dysfunction, inflammation, and oxidative stress. Genus Boswellia is a genus in the Burseraceae family. It comprises several species traditionally used for treatment of chronic inﬂammatory diseases, cerebral edema, chronic pain syndrome, gastrointestinal diseases, tumors, as well as enhancing intelligence. Many studies have been carried out to discover therapeutic approaches for neurodegenerative diseases such as Alzheimer’s diseases, Parkinson’s disease, Huntington’s disease, multiple sclerosis and amyotrophic lateral sclerosis, stroke, and concomitant cognitive deficits. However, no curative treatment has been developed. This paper provides an overview of evidence about the potential of the Boswellia species and their main constituents, boswellic acids, as modulators of several mechanisms involved in the pathology of the neurodegenerative diseases. In vitro, animal, and clinical studies have confirmed that Boswellia species contain bioactive components that may enhance cognitive activity and protect against neurodegeneration. They exert the beneficial effects via targeting multiple pathological causes by antioxidative, anti-inflammatory, antiamyloidogenic, and anti-apoptotic properties. The Boswellia species, having neuroprotective potential, makes them a promising candidate to cure or prevent the neurodegenerative disorders.


Introduction
Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), multiple sclerosis (MS), amyotrophic lateral sclerosis, and stroke are age-related disorders (1,2). A number of common pathophysiological features have been proposed for these diseases, including elevated oxidative/nitrosative stress, mitochondrial dysfunction, protein misfolding/aggregation, synapse loss, and decreased neuronal survival (3,4). Considering limitation of effective treatments for these diseases, there is an urgent need for new strategies using natural products that act through novel biological targets (5 (6,7). The genus is widespread in dry areas such as Arabia, northeastern coast of Africa, and India (8). The species have been useful in traditional medicine for treatment of inflammatory diseases, including asthma, arthritis, cerebral edema, chronic pain syndrome, gastrointestinal disease, tumors, and for enhancing memory and learning function (9)(10)(11). Frankincense, oleo-gum resins obtained from the genera Boswellia, is composed of essential oil (5-9%), mucopolysaccharides (20-23%), and resin (60%) (12,13). The resinous part contains tetracyclic and pentacyclic triterpene acids. Boswellic acids (BAs) are considered the main biologically active components among the triterpene acids ( Figure 1) (8,14). Frankincense is responsible for anti-inflammatory and anti-cancer effects of BAs (15,16). The anti-inflammatory mechanisms are applied through inhibition of 5-lipoxygenase, cathepsin G, and microsomal prostaglandin-E synthase (mPGES)-1 (17). Other mechanisms include suppression of nuclear transcription factor κB (NF-κB) and pro-inflammatory cytokines such as tumor necrosis factor (TNFα), interleukin (IL)-1β, IL-2, and IL-6 (15,17). Also, BAs lead to induction of apoptosis in cancer cells via activation of caspase-8 and inhibition of topoisomerases-I and II-alpha (16,18). In this review, the therapeutic effects of Boswellia and their major constituents on various neurodegenerative disease models have been summarized ( Figure 2). Herein, pharmacological effects of the genus Boswellia in neurodegenerative diseases were classified as follows: 1. Alzheimer's disease 2. Parkinson's disease 3. Cognitive dysfunction 4. Multiple sclerosis 5. Central nervous system trauma and brain ischemia

Alzheimer's disease
Alzheimer's disease is the most common type of neurodegenerative dementia in older people (19). It is characterized by amyloid-beta (Aβ) accumulation in plaques and hyper-phosphorylation of tau protein forming neurofibrillary tangles (20). Aβ aggregation and neurofibrillary tangles induce neuron and synapse loss and gross degeneration in the temporal lobe, parietal lobe, as well as parts of the frontal cortex and cingulate gyrus (21). The pathological alterations cause progressive memory loss, cognitive impairment and the inability to perform daily activities (21,22). Aβ toxicity, cholinergic dysfunction, oxidative damage, apoptosis, synaptic dysfunction, and senile plaque-induced inflammation have been postulated to be involved in pathogenesis AD (21,23). The possible prophylactic and therapeutic effects of B. serrata using an animal model AD induced by AlCl 3 (17 mg/kg for 4 weeks, orally) were assessed. In this study, rivastigmine (0.3 mg/kg/day), as standardized medicine, and B. serrata (45 and 90 mg/kg/ day) were given for 2 weeks before AlCl 3 administration to rats. The results revealed that activity of rats increased, while the duration taken by rats to reach food in the T-maze test decreased. According to biochemical analysis, treatment with B. serrate led to elevation of acetylcholine (ACh) levels while acetylcholine esterase (AChE) activity was suppressed in brain homogenates. The histopathology findings indicated that amyloid plaques reduced in the hippocampus (24). In a preclinical investigation, therapeutic potential of B. serrata against neurodegeneration using an AlCl 3 -induced rat model of AD was claimed. Following treatment of AD animals with B. serrata as resin methanolic extract (137.5 mg/ kg, 3 months, orally), Aβ plaques in histopathological samples disappeared. Biochemical analysis showed brain and serum levels of AChE, C-reactive protein (CRP), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), monocyte chemoattractant protein-1 (MCP-1), and leukotriene B4 (LTB4) were suppressed while brain ACh and Bcl-2 were elevated. The data represented preventing efficacy of B. serrata against neuro-inflammatory and apoptosis insults (25). Also, co-administration of ginger (Zingiber officinale, 108 and 216 mg/kg) and B. serrata (45 and 90 mg/kg) in rats treated with AlCl 3 . The B. serrata and ginger improved histopathologic changes and also behavior stress tests, including activity cage, rotarod, T-maze, as well as restoring ACh and AChE levels in brain homogenate (26). Recent evidence revealed that insulin resistance and metabolic dysfunction play an important role in the pathology of sporadic Alzheimer's disease (sAD) (27). Intracerebral-ventricular injection of streptozotocin (STZ, 2-deoxy-2-(3-(methyl-3nitrosoureido)-D-glucopyranose) is applied to mimic sAD (28). STZ -induced insulin resistance causes several features characterizing AD including oxidative stress, neuroinflammation, and dysfunctions in adult neurogenesis that are followed by progressive deficits in learning and memory (29)(30)(31). A study explored whether aqueous extract of frankincense from B. carteri could have therapeutic effects on STZ-induced memory impairment. The evaluation of learning using passive avoidance task (PAT) indicated that chronic administration of aqueous extract of frankincense (50 mg/kg, 42 days) improved memory in rats receiving STZ (1.5 mg/kg/2 μl/side, i.c.v.) in a time-dependent manner (32). SuHeXiang Wan (SHXW) is a traditional Chinese medicine comprising Liquidambar orientalis, Saussurea lappa, Aquilaria agallocha, Santalum album, B. carteri, Eugenia caryophyllata, Cyperus rotundus, Styrax benzoin, and Dryobalanops aromatica that has been used orally for the treatment of seizures, infantile convulsions, and stroke (33). The potential beneficial effects of SHXW essential oil were investigated on SH-SY5Y neuroblastoma cells and animal AD model induced by Aβ1-42 in mice. SHXW essential oil attenuated Aβinduced cytotoxicity in SH-SY5Y cells through inhibition of apoptosis and ROS generation. Up-regulation of heme oxygenase-1 (HO-1), nuclear factor erythroid 2-related factor (Nrf2) expression, and increased Bcl-2/Bax   (97).
Assessment of effect on neuro-recovery following diffuse axonal injury in a double-blind, randomized, Enhanced the cognitive outcome of patients with diffuse axonal injury protein ratio have been shown to be involved in the protective effects (34). SHXW essential oil ameliorated cognitive dysfunction in Aß1-42 treated mice, and it was associated with reduced p38, c-Jun N-terminal kinases, and tau phosphorylation (34). The findings suggested SHXW essential oil as a therapeutic agent for the prevention and treatment of AD and other tau protein pathology-related neurodegenerative diseases (34). Collectively, the experimental studies confirmed the inhibitory potential of Boswellia against formation of amyloid plaques and degeneration of cholinergic neurons induced by Aß. The medicinal herbs were found to induce anti-apoptotic activity through modulation of Bcl-2/Bax protein ratio. In addition, they counteracted oxidative damages through enhancement of HO-1/ Nrf2 protein expression and restoring oxidative stress markers (Table 1).

Parkinson's disease
Parkinson's disease (PD), a common chronic, progressive neurodegenerative disorder of the elderly, is characterized by motor (including bradykinesia, tremor, and rigidity) and non-motor symptoms (35). The symptoms of PD result from the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and dopamine (DA) deficiency in the striatum (36). Moreover, the presence of α-synuclein containing Lewy bodies in the surviving neurons is also proposed in the neuropathology of PD (37). Oxidative stress, mitochondrial dysfunction, excitotoxicity, calcium cytotoxicity, trophic factor deficiency, inflammatory processes, genetic factors, and apoptosis are now considered to be key mechanisms that contribute to neurodegeneration in PD (36). Some evidences has demonstrated the neuroprotective potential of B. serrata on dopaminergic neurons that can be applicable in PD. Boswellia resin extract (10 µg/ ml) attenuated MPP+ (1-methyl-4-phenylpyridinium, 1000 µM), an active metabolite of 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyryridine (MPTP)-induced toxicity in human dopaminergic SK-N-SH cell-line. The protective effects were associated with increased cell viability and reduced apoptotic features (38) (Table 1).

Cognitive dysfunction
Learning means the process of acquiring knowledge from the outside environment while memory is retention and retrieval of learned information at a later date (39-41). Short-term plasticity (STP) and long-term potentiation (LTP), two types of synaptic plasticity, are mechanisms for memory storage (42)(43)(44)(45). Learning and memory impairment are considered the most significant features of dementia (46). A number of experimental studies were conducted to evaluate the effect of maternal administration frankincense (an oleo-gum resin derived from trees of genus Boswellia) on the cognitive capabilities. Hosseini Sharifabad et al. assessed learning and memory in two-month-old male Wistar rats whose mothers orally received aqueous extract of B. serrata (0.1 g/kg/day) during gestation (3 weeks) using active avoidance learning test. Frankincense enhanced power of learning at post-learning stage, short-term memory, and long-term memory (47). The results were relevant to the alteration in the neurites of CA3 hippocampal cells reported in another study. Analysis of morphology of dendritic architecture of CA3 hippocampal neurons indicated more dendritic segments and branching density in young rats whose mothers were treated with B. serrata (100 mg/kg/day) during gestation compared with untreated rats (48). Administration of aqueous extract of frankincense (50 and 100 mg/kg, 4 weeks) facilitated the learning and spatial memory formation in rats. The results were demonstrated as reduction in escape latency and traveled distance by the Morris water maze test method (49). An in vivo study was performed to assess the efficacy of frankincense for memory formation during development of the rat brain. For this purpose, aqueous extract of frankincense (50 and 100 mg/kg) was orally administered into female rats during gestation and lactation periods. Memory performance and hippocampal calcium/calmodulin kinase II (CaMKII) and CaMKIV mRNA levels in the offspring rats were evaluated to identify potential molecular change during gestation and lactation periods (50). CaMKII and CaMKIV are involved in many signaling cascades and are thought to be crucial mediators of learning and memory (51). CaMKII, as an important component of the postsynaptic density of glutamatergic synapses (52), plays a role in regulation of synaptic transmission and induction of long-term potentiation (LTP) (53).
Continued Table 1 Ach According to the results, up-regulation of CaMKII-α mRNA expression of the hippocampus was concomitant with improvement of spatial memory retrieval in offspring rats (50). Evaluation of the spatial memory parameters by the Morris water maze (MWM) test revealed improvement of spatial learning and memory in rats treated with aqueous extract frankincense (50 and 100 mg/kg/day for 4 weeks). Frankincense up-regulated expression of brain-derived neurotrophic factor (BDNF) transcripts but not cAMP response element-binding (CREB). Therefore, the effects of the extract on memory formation may be attributed to another BDNF-related pathway other than BDNF-CREB-BDNF cycle (54). B. papyrifera total extracts (300 mg/kg, three times a day, orally) and boswellic acids fraction (100,200, and 300 mg/kg) enhanced the retention phase of spatial memory of adult male rats in the MWM task. The results of the investigation proposed improving potential of Boswellia and boswellic acid fraction in memory function in normal subjects or neurodegenerative disorders (55). Impairment of cognitive function including memory, visuospatial organization, attention, and reaction time in overt hypothyroidism has been recognized for more than a century (56)(57)(58). Olibanum (resin of B. serrata) exhibited beneficial effects on memory deficit in methimazole-induced hypothyroidism model. Oral administration of olibanum (100 and 500 mg/kg, 180 days) improved memory and learning impairment in hypothyroid rats by the Morris water maze test (59). Animal models of amnesia induced by scopolamine are widely used to screen potential therapeutic value of compounds in treatment of dementia (60). Another study aimed to assess the effect of ethyl acetate and N-butanol fractions of B. carterii gum resin on intact memory and hyoscine-induced memory impairments using the MWM task. Ethyl acetate (0.1 mg/kg) and N-butanol (0.1 mg/ kg) fractions remarkably enhanced intact memory. The ethyl acetate fraction was much more significant than other fractions in enhancing the memory (61). The combination of Melissa officinalis and B. serrata improved scopolamine-induced cognitive impairment. The MWM method revealed co-administration of M. officinalis and B. serrata (200 and 400 mg/Kg body weight) before scopolamine injection (0.1 mg/kg) led to improvement of memory function (62). Neuro-inflammation can cause cognitive deficits since it affects memory processing during consolidation and retrieval stages (63,64). Considering anti-inflammatory activity of frankincense has been approved with an (15). Lipopolysaccharide (LPS) triggers the neuro-inflammatory process through activation of nuclear factor kappa B (NF-κB) pathway in microglia in the central nervous system (CNS) (65,66). Administration of hydro-alcoholic extract of frankincense (50 mg/kg; orally) before LPS (1 mg/kg; IP) enhanced step-through latency (STL) in a passive avoidance task (PAT) via decreasing the TNF-α level in the hippocampus. Therefore, anti-inflammatory effects of frankincense may be involved in inhibition of memory loss (67). Clinical and pre-clinical studies have shown that prolonged frequent seizures cause cognitive, memory, and emotional impairments (68). These recurrent seizures affecting the hippocampus may lead to cell damage and death in the cornu ammonis (CA1) region (69). Function of CA1 neurons in the hippocampus plays a vital role in converting short-term memory to long-term memory (70). Pentylenetetrazol (PTZ)-induced kindled rats animal model was used for evaluation of epilepsy and its consequences on memory (71). The aqueous extracts of B. serrata (0.1, 0.5, and 1 g/kg, IP) improved passive-avoidance learning ability in kindled animals indicated by using shuttle box apparatus and step-through latency method. The findings were associated with increasing number of pyramidal neurons and dendritic spines in CA1 (71,72). Therefore, consumption of B. serrata may be a therapeutic strategy for decreasing harmful effects of seizures on cognitive function (71). Age-related spatial learning deficits have been suggested to be due to changes that appear mostly in hippocampal connectivity and plasticity (73). The three main fields of the hippocampal region, CA1, CA3, and particularly dentate gyrus are vulnerable to aging (73). An experimental study conducted by Hosseini-Sharifabad et al. investigated the effects of chronic administration of B. serrata hydroalcoholic extract (BSE) on the learning performance and the morphology of hippocampal granule cells in aged rats (74). The rats (24 months old) received the aqueous extract of BSE (100 mg/kg/d, for 8 weeks, intragastrically), after this time, dendritic complexity in the dentate granule cells and spine density on the dendritic tree of the cells increased (74). These findings were observed along with improvement of spatial learning capability indicated as decrease in escape latency and swimming distance (74). Neuroprotective potential of Boswellia resin in age-related morphological changes and concomitant cognitive deficits may suggest it as a therapeutic agent in neurodegenerative diseases (74). In a randomized, parallel, double-blind, placebo-controlled clinical trial, administration of B. serrata and Melisa officinalis extracts (290 mg and 27 mg, for a month) improved memory in 70 older adults (75). Overall, this evidence provides preliminary support for the cognitive-enhancing efficacy of genus Boswellia. Potential beneficial actions may be attributed to BDNF up-regulation (Table 1).

Multiple sclerosis
Multiple sclerosis (MS) is a chronic autoimmune, inflammatory neurological disease of the CNS, which leads to the destruction of myelin, oligodendrocytes, and axons (76). Quinolinic acid (2, 3-pyridine dicarboxylic acid), a neuroactive metabolite of the kynurenine pathway, is an agonist of N-methyl-D-aspartate (NMDA). Inappropriate activation of the kynurenine pathway may increase quinolinic acid levels, which is often implicated in the pathogenesis of a number of neurological diseases such as MS (77,78). In vitro study revealed that 24 hr pre-treatment of oligodendroglia (OLN-93) cells with ethanolic extract of B. serrata oleo-gum resin (10,20,40, and 80 µg/ml) prior to glutamate exposure reduced glutamate and quinolinic acid-induced oxidative injury (8 mM) (79). A mixture extract of Portulaca olerace, Urtica dioica, and B. serrata (200 and 400 mg/kg) has protective effects against ethidium bromide-induced MS model (80). The results revealed neurogenesis and memory improvement using the shuttle box test following treatment with the mixture (81). Cognitive deficits have been reported in up to 70% of MS patients (80). A clinical trial study was carried out in 80 patients with relapsing-remitting MS. In this randomized, doubleblind, placebo-controlled study, effect of B. papyrifera on cognitive impairment in MS patients was investigated using brief international cognitive assessment for MS (BICAMS), symbol digit modality test (SDMT), and the California verbal learning test (CVLT). The patients received B. papyrifera (300 mg capsule, twice a day) and placebo with the same dose for 2 months. B. papyrifera remarkably improved visual-spatial memory while it was not effective in verbal memory and information processing speed, which may be due to the prescribed dose (82). Another double-blind clinical trial was carried out in 60 MS patients with cognitive deficits to assess the efficacy of B. serrata extract in treatment of cognitive dysfunction. The patients categorized randomly into two groups of treatment and placebo received 450 mg of B. serrate extract or placebo capsules, respectively, twice a day for two months. The extract remarkably enhanced auditory/verbal and visual/spatial memory using the brief visuospatial memory test (BVMT) and CVLT compared with the placebo group, which confirmed potential of B. serrata for MS patients suffering from  Boswellia in neurodegenerative diseases Rajabian et al. cognitive impairments (83) ( Table 1).

Central nervous systems trauma and brain ischemia
Stroke is the fourth cause of death and one of the main causes of disability worldwide (84). Ischemic stroke is the most common type of stroke, accounting for about 80 percent of all strokes, which results from transient or permanent cessation of cerebral blood flow (85,86). Following brain ischemia, the level of glutamate increases, leading to over-activation of its receptors, including NMDA receptors and raised intracellular calcium (87). Brain ischemia also can trigger inflammatory responses and subsequently neuro-inflammation. Therefore, strategies targeting these pathways involve NMDA receptor antagonists, calcium channel blockers, and anti-inflammatory and antioxidant agents, which may be used as prophylactic or therapeutic for ischemia damage of brain tissue (87,88). B. serrata and its constituent, AKBA (3-acetyl-11-keto-β-boswellic acid), exhibited potential neuroprotective and anti-oxidant activity (89). Neuroprotective potential of BSE and AKBA against ischemia-induced cytotoxicity was investigated. The survival of PC12 neural cells, pretreated with BSE (1.5-6 µg/ml) and AKBA (0.5-2.5 µg/ml) for 2 hr before exposure to oxygen/glucose/serum deprivation (OGSD) condition, increased. Moreover, BSE and AKBA counteracted oxidative stress indicated as restoring of intracellular reactive oxygen species content, lipid peroxidation, and oxidative DNA damage (90). Pre-and co-treatment with BSE and AKBA prevented glutamateinduced PC12 and Neuro-2a cell toxicity. The protective effect may be related to their inhibitory effects against oxidative damage and apoptotic cell death (91). An in vitro investigation aimed to explore anti-glycation and anti-oxidant potentials of B. sacra oleo-gum resin. Dichloromethane (CH 2 Cl 2 ) fraction of the resin, 40% dichloromethane (CH 2 Cl 2 )/n-hexane sub-fraction, and frankincense oil exhibited l,l-Diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity. In addition, moderate superoxide anion scavenging activity was exhibited by polar fraction, while the highest antiglycation activity for polar fractions were reported (92). Another study was designed to investigate phytochemical screening, in vitro antioxidant activity of leaf extract of B. serrate. The methanolic extract of B. serrata contains alkaloids, terpenoids, saponins, and flavonoids (93). Methanolic extract exhibited significant DPPH free radical scavenging activity (IC 50 = 54.06 µg/ ml) and ferric reducing power (IC 50 = 62.12 µg/ml), in a dose-dependent manner (93). Administration of the aqueous and ethanolic extracts of B. serrata (125, 250, and 500 mg/kg, IP) and AKBA (50 mg/kg, IP) just after middle cerebral artery occlusion (MCAO), for 30 min and reperfusion for 24 hr improved neurological deficits and reduced brain infarction volume. The extracts diminished neuronal apoptotic cell death through up-regulation of Bcl-2 and down-regulation of Bax and caspase-3. The modulated cerebral redox status was also indicated as inhibition of lipid peroxidation while increasing glutathione content and superoxide dismutase activity in the cerebral cortex (94). The neuroprotective effects of Boswellia against brain stroke were further confirmed by the reduction of infarction volume and neurological impairments. The aqueous extract of frankincense was administrated (50, 100, and 150 mg/kg, orally for 30 days). Two hours after the last treatment with frankincense extract, the rats were subjected to MCAO for 60 min followed by reperfusion for 24 hr. The level of blood-brain barrier (BBB) permeability and strokeinduced brain edema decreased in rats treated with aqueous extract of frankincense at doses of 100 and 150 mg/kg (95). The results of a prospective, randomized, placebo-controlled, double-blind, pilot trial confirmed the efficacy of BS on cerebral edema following brain radiotherapy (96). In this trial, forty-four patients with primary or secondary malignant cerebral tumors randomly received BS (4200 mg/day) or placebo during radiotherapy. Administration of BS suppressed the edema volume which was evaluated by T2-weighted magnetic resonance imaging (MRI) (96). To investigate the effect of B. serrata on neuro-recovery following diffuse axonal injury (DAI), a double-blind, randomized, cross-over study was designed. The outcome of diffuse axonal injury was assessed using the disability rating scale, a surrogate clinical marker for the pace of neurorecovery. 38 patients randomly received either placebo or BS capsules (containing 215 mg BS gum resin) for 6 weeks. The BS resin enhanced the cognitive outcome of patients with DAI (97). The protective effects of the condor against cerebral inflammation after induction of diffuse traumatic brain injury were investigated. B. serrata (250 and 500 mg/ kg) attenuated brain edema and disruption of blood-brain-barrier induced by traumatic brain injury. The results were accompanied by improvement of vestibulomotor dysfunction as well as modulation of IL-1β and I-10 in brain tissue. Antiinflammatory properties were suggested to be involved in the neuroprotective effects (98). Boswellia exhibited therapeutic potential for brain ischemia and injuries, which is most likely related at least in part to its antiinflammatory, anti-apoptotic, as well as anti-oxidative and free radical scavenging activities (Table 1).
In this review, Tables 1 and 2 represent the brief description of pre-clinical and clinical studies on protective effects of genus Boswellia and AKBA in the neurodegenerative diseases, respectively.

Conclusion
Considering lack of effective therapy for clinical applications, pharmacologically active natural products, having neuroprotective activities are being focused, which makes them potential candidates for neurodegenerative disorders. The genus Boswellia has been suggested to target various molecular pathways involved in pathogenesis of neurodegenerative diseases. The genus regulates neurotrophic factors (including BDNF), apoptotic proteins (pro-apoptotic caspase-3 and anti-apoptotic bcl-2), and redox status. They were shown to be therapeutically effective at controlling inflammatory and cholinergic systems. Therefore, evidence suggests the importance of the genus in the prevention and treatment of neurodegenerative diseases even though further studies and clinical trials on these promising medicinal plants and their constituents should be strongly encouraged in the future.